Particularly for antenna design purposes—but not only there—difficulties in some cases occur in providing links or connections without any intermodulation. This problem occurs in particular at interfaces to which different assemblies are intended to be connected, as required.
Radio-frequency connections between two radio-frequency assemblies, for example between a radio-frequency board and a wire-free transmission device, for example antennas, are normally provided by means of coaxial connection techniques. However, disadvantageous and undesirable intermodulation can also occur here. Improvements to avoid or to reduce passive intermodulation when using coaxial plug connections have been proposed, by way of example, in U.S. Pat. No. 6,414,636 B1. However, if the aim, for example, is to connect a specific distribution network for a so-called smart antenna, as is known in principle from U.S. Pat. No 6,463,303 B1, in order to produce a specific polar diagram characteristic for the antenna under discussion, then the costs for a module which can be connected in such a way furthermore also rise considerably if all the connections on the input and output side are in the form of coaxial plug connections.
Thus, in principle, it would also be possible to provide capacitive connections instead of coaxial plug connections.
Capacitive RF connections have been disclosed, for example, in U.S. Pat. No. 5,812,037. These have a stripline filter coupling structure, which operates capacitively.
A PCMCIA signal connector, as is normally used for Notebook Computers, has in principle been disclosed in U.S. Pat. No. 5,936,841. The PCMCIA plug-in board normally has a male connector strip on one of its end faces, which interacts with a male connector strip which is integrated in the Notebook, when the corresponding PCMCIA board is inserted into a holding slot in the Notebook. A first electrically conductive layer, which represents one half of the RF coupling device, is then provided on one of the large side surfaces, parallel to this side surface. The second electrically conductive layer, which is parallel to the first, is accommodated with a lateral offset in the interior of the apparatus. There is an air gap (resulting from the lateral distance between the PCMCIA board and the adjacent inner boundary surface of the plug-in slot for the electrical apparatus, for example in the form of Notebook) and dielectric intermediate layer, which is part of the wall of the Notebook, between the two conductive layers of the RF coupling structure which are parallel to one another.
However, the undesirable intermodulation cannot be avoided even by means of a capacitive RF connection for a PCMCIA board such as this.
The object of the exemplary illustrative non-limiting technology described herein is thus to produce a radio-frequency connection and, in particular, a radio-frequency distribution network, which can be connected as required to an interface that is provided, with the aim of largely avoiding or precluding intermodulation.
The production of a floating radio-frequency connection at an appropriate interface allows a modular link without any intermodulation, for example between an RF network and a basic module. In this case, not only the signal lines which carry the signal but also the outer conductors or earth conductors are capacitively connected to one another at the corresponding contact devices, while avoiding any conductive contact. The nature of the interface in the form of the capacitive coupling via an interface with contacts has the major advantage of a low level of intermodulation, as is actually of major importance for mobile radio applications, such as mobile radio antennas. If very strong intermodulation products occur in the transmission frequency band, and whose frequencies extend into the reception frequency band, it would no longer be possible for mobile devices such as mobile telephones to receive weak signals at these reception frequencies.
The fact that a modular link between an RF network having two or more connections or connecting points to an RF device, for example a mobile radio antenna, can be achieved without any intermodulation on the basis of this principle is in this case surprising for a number of reasons. This is because it would necessarily have been presumed that, when forming corresponding coupling surfaces running parallel to one another and on which the respective RF signal is intended to be transmitted, or else for producing the floating earth connection, further influences would be noticeable which would make it impossible to produce an RF coupling connection which could always be reproduced unambiguously. This is also due to the fact that, especially when using mobile radio antennas or transmission antennas, it is absolutely essential to use a metallic housing for screening purposes. However, metallic housings fundamentally have effects on the electrical conditions and characteristics if capacitive coupling devices are used in the interior of the screened housing. This is because, in some circumstances, the distance between the coupling surfaces and the screening housing results in an additional parasitic parallel capacitance between the coupling surfaces and the electrical earth.
However, an exemplary illustrative non-limiting design of the RF connecting device also makes it possible to minimize these effects and influences.
The geometry of the coupling surfaces governs the electrical parameters for signal transmission, such as the matching to the characteristic impedance (VSWR), the insertion loss and the bandwidth of the frequency band. In order to improve fine tuning further, one preferred development of the exemplary illustrative non-limiting implementation also provides, for example, for the coupling surface on a board that is used to be provided with “small tabs” or so-called “extension surfaces”, which project at the sides. These small tabs or extension surfaces, in parallel with the coupling between the coupling surfaces, produce an additional small amount of coupling between the coupling surfaces on a board and an earth surface.
The exemplary illustrative non-limiting network module, which can be coupled to a basic module, furthermore has capacitively coupled earth surfaces, in addition to the coupling surfaces which provide capacitive RF coupling, in order to suppress the intermodulation-free modular link. This metal structure, which covers the board, is preferably formed on the face on which the corresponding electrical earth surfaces of the basic module are located. In this case, an insulating film with a predefined thickness is preferably used for insulation between the two electrical earth surfaces which produce the earth coupling. The coupling surfaces of the electrical earth surfaces which provide the signal transmission and which in some cases are also referred to in the following text as coupling fingers are in contrast to this preferably formed on the opposite face of the board of the network module, so that the substrate of the board acts as insulation for the corresponding signal coupling surface on the basic module.
The radio-frequency network on said board may, for example, be based on stripline technology (microstrip technology).